In seismic exploration for oil and gas, an energy source such as an explosive or a large vibrator imparts an elastic deformation to the earth which propogates radially downward and outward through the earth formations. At levels where there is a significant change in acoustic impedance of rock layers, a portion of the energy is reflected and travels back upward to the surface, where minute movement of earth particles occur at the free boundary between the earth and the atmosphere.
A seismic detector, commonly refered to as a geophone, positioned at the ground surface is used to convert particle motion into an electrical output signal. Typically a number of such geophones are laid out in a line with individual phones being connected together by leader cable sections to provide a group of 6-24 phones (usually 12) in a string. The leader is electrically connected to a recording instrument, and after passing through suitable signal conditioning apparatus the data is recorded on magnetic tape for subsequent processing and interpretation.
The length of the leader cable sections determines the geophone spacing, which in the United States Gulf Coast area normally is a 25 foot separation. On the other hand, in West Texas the geophones may be replaced 50-60 feet apart in order to provide more effective cancellation of undesirable horizontally propogating interference signals of longer wave lengths. In any event the geophysical crew that conducts the survey is faced with a number of practical problems in properly laying out the geophone strings, and in servicing strings by replacement of leader cable sections which have been damaged, for example, in handling. The leader typically has three insulated conductors that are twisted together and enclosed in a common jacket.
Other problems that are encountered involve properly anchoring the geophones to the leader cable in a manner to protect the integrity of electrical continuity of the cable conductors and the geophone terminals, providing a protection for the geophones from the environment, and insulation from ground. Of course it will be recognised that the motions of the earth may be minute, and consequently the geophones have a very low signal output level, such that AC power lines in the area can provide an interference which recuces the signal-to-noise ratio in a highly undesirable manner. In fact, one of the biggest problems that faces the seismic field crew is the maintenance of a high level of electrical isolation between the signals generated by the geophones, and interferring electrical noise that is present in the area being surveyed. Particularly troublesome is the 60 Hz noise radiated by the grid of electrical power lines that cover most of the United States.
Pickup of electrical noise signals by the leader wire, and the internal electrical circuits of the geophones themselves, is greatly amplified by the presence of moisture. If moisture gets into the geophone case, or past the conductor insulation through cuts or holes in the jacket and the insulation, the use of the string is considerably impaired and often renders the seismic data unusable. As a result, new and better ways have been continually sought to prevent any electrical leakage to earth ground of the electrical signals generated by the geophones and carried by the leader wire or cable.
Users also are continually seeking better ways for servicing and repairing faulty geophone strings. Faults develop due to defective geophones, pieces of leader wire becoming cut or broken, and from both geophones and wires being damaged through inadvertent encounters with heavy field equipment and harsh environments. No matter how sturdy or rugged the geophone strings may be, and no matter how much care is exercised by the field crew in handling the equipment, damage and breakdown does occur so that field repairs must be made.
A known method of facilitating replace of defective geophones involves the use of plug-in connectors that are molded to the leader cable at each geophone station. This method permits easy replacement of defective phones by opening the case and then simply unplugging the element and plugging in a new element in its place. However, there are some serious disadvantages to the use of this technique. This type of repair can be quite costly, and it may not be possible to make field repairs or replacements of the plug-in connector which is molded to the geophone leader cable. Thus where a serious break or cut occurs in a section of the leader cable, it was often necessary to throw away the entire length of leader, including the expensive connectors which are molded thereto.
Another disadvantage of the molded plug-in connector is the relative inflexibility of such an arrangement with respect to spacing. If the user encounters a prospecting or survey area where the desired spacing between adjacent geophones of a string is greater that the distance between the molded connectors on the leader, there is as a practical matter no way to overcome the problem. Such inflexibility also is a problem if the user needs to change the electrical interconnection between geophones in a string. This means that the geophone spacing must be selected prior to molding of the plug-in connectors, and once the string is fabricated, the electrical hook-up can not be changed.
In view of the disadvantages of the molded plug-in connector approach discussed above, this technique is no longer favored by users and has been substantially abandoned.
As mentioned above, another significant problem area has been the difficulties encountered in obtaining an adequate seal around the solder joint connections between the geophone terminals and the leader conductors so that the phone is isolated from the atmosphere. If the seal is inadequate, then any moisture that enters the case can cause electrical leakage to ground and the generation of spurious electrical noise. On the other hand, if the solder joint is potted in a sealing compound, it becomes very difficult to unsolder the connections, which is necessary when a defective geophone or length of leader wire is to be replaced. Various techniques have been tried in efforts to solve this problem, with perhaps the most widely used technique being to pot the connections in a silastic rubber compound that is more easily removed than certain other types of potting compounds. However, the seal against moisture entry is not as effective as would be desirable, and on-site repotting by field crews after the making of repairs is a source of additional problems.
Another prior sealing technique uses gaskets rather than potting compounds, and this technique has gained some degree of favor. However, gaskets do not provide a seal for the basic geophone element itself, and moisture that enters the case via a damaged leader wire, or any other source, can cause electrical leakage and, in more severe cases, rusting of the geophone element.
It will be recognized from the foregoing discussion that prior methods for stringing geophones have encountered a sizeable number of problems, particularly when it is necessary for the seismic crew itself to make field repairs, regardless of whether the problem is a damaged leader section, a faulty geophone element, or a damaged case. In any event it has been necessary for the field crew to open the geophone case and then make a repair that is appropriate to the problem. Experience has taught users that the necessity for a crewman to open the case as an incident to repair is not satisfactory. The geophone is a precision instrument that must be electrically connected in the string in a precise, correct number. The repaired or replaced geophone must not be connected in a manner that produces reversed electrical polarity, and the case must be repotted or a sealing gasket installed in a manner such that dirt or other foreign matter which could lead to a poor seal and electrical leakage is not introduced. As can be readily appreciated, the typical seismic field crew would normally lack the expertise and technical knowhow necessary to make an adquate field repair, and effective and efficient repair facilities are usually not available to a crew that makes frequent moves from one location to another. Therefore, problems have continued to exist in prior art methods and techniques for effecting repairs on geophone strings.
It is the general object of the present invention to provide a new and improved geophone apparatus that obviates the foregoing problems.